For astronomers who study cosmic evolution, the past six months have brought unprecedented revelations. Data from ground- and space-based observatories have revealed what appear to be the remnants of the first stars ever to shine. They also have pinned down the age of our universe and outlined its bulk composition.
Cosmology "has turned a corner," says Charles Bennett, at NASA's Goddard Space Flight Center in Greenbelt, Md. Astronomers no longer have to speculate about some of the earliest crucial events in cosmic history. They can check their theories of what happened after the universe appeared in the so-called "big bang" explosion of energy against what they actually see in the sky.
The excitement began last Oct. 31, when Norbert Christlieb, from Uppsala University in Sweden, and eight colleagues published, in Nature, their discovery of the oldest star yet found in our Milky Way galaxy. Given the prosaic name HEO107-5240, it lies 36,000 light-years away in the galaxy's halo of ancient stars. It has four-fifths the mass of our sun.
The excitement continued with the European Space Agency's announcement April 30 that a team led by German astronomer Wolfram Freudling had found a record of the "ashes" of the first generation of stars in Hubble Space Telescope data.
What astronomers are looking for: the presence and abundance of metals in the very young universe. By "metal" they mean any element heavier than the hydrogen and helium that formed from the energy released in the original big bang. Many cycles of this process - in which all other elements are created within stars and ejected into space - produce metal-rich stars like our sun.
The Christlieb team says the ancient star it found is so poor in metals that it probably is the daughter of the first stellar generation. If so, those original stars lit up very early in cosmic history. Subsequent discoveries support that possibility.
Astronomers have been looking for clues in distant quasars, which are very energetic young galaxies. A few weeks ago, Ian Robinson, with the UK Astronomy Technology Center in Edinburgh, reported that a British team using the James Clerk Maxwell Telescope in Hawaii has found massive amounts of metal-rich dust in one of those quasars. Then Dr. Freudling's team reported having discovered iron in others. Given the youth of these quasars - about 900 million years after the big bang - both scientific teams conclude that the stars that produced the dust probably formed when the universe was only 200 million years old.
That's far earlier than astronomers once thought possible. Yet it's consistent with what a NASA satellite is finding in radiation left over from the original big bang. Tiny variations in that radiation today reveal details of early conditions. As reported at a Washington press conference in February, those details now show that the universe is 13.7 billion years old, with an uncertainty of only 1 percent. It is composed of 4 percent ordinary matter, 23 percent of an unknown material called "dark matter," and 73 percent of mysterious dark energy. The details also indicate primordial stars did indeed ignite when our universe was only 200 million years old.
Cosmic history still raises many questions. But when it comes to the big one - is the concept of a big-bang origin valid - the data now say "yes."